The Oxford-Family Planning Association contraceptive study has now followed 17,000 women, predominantly of childbearing age, for a total of more than 200,000 woman-years. The incidence of myocardial infarction and angina in women aged less than 50 years has been low: 0.03/1,000 woman-years at ages 25-34 rising to 0.67/1,000 woman-years at ages 45-49. However, the overall incidence in women who were smokers at entry to the study is more than three times that in women who were non-smokers, the increase in individual risk being proportional to the number of cigarettes smoked. Observations on other risk factors must be treated with caution in view of the small numbers involved: in general, the differences and trends reported are not statistically significant. However, a consistent positive relation is observed between incidence rates and both relative weight and parity after adjustment for age and smoking, while no consistent trend is observed for social class. Ever use of oral contraceptives is associated with a twofold increased risk of myocardial infarction (not statistically significant), but there is no increased risk in current users as was suggested by earlier studies. This may reflect the adoption of lower dose preparations and the positive selection of healthy women for oral contraception.
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